Methane from Syngas by Anaerobic Digestion Sanjay Shah Wenche Hennie Bergland Rune Bakke Department of Process, Energy and Environmental Technology, University College of Southeast Norway, Norway [email protected], [email protected], [email protected] Abstract Several techniques are being used for biogas Anaerobic digestion (AD) is a prominent green upgrading like water washing, polyglycolic adsorption, technology used for methane production from organic pressure swing adsorption and chemical treatment waste. Previous studies have shown that the amount of (Osorio & Torres, 2009). These methods are performed outside of the anaerobic reactor for biogas upgrading CH4 produced during anaerobic digestion can be increased by adding inorganic electron donors such as which requires extra investments. Previous studies have shown that CH4 in AD can be increased by adding H2 and CO, both which can be produced as syngas from wood. Syngas inflow is implemented in the ADM1 inorganic electron donors such as H2 and CO (Luo & model and simulations are carried out with different Angelidaki, 2013). These can, for example, be produced syngas additions to a well-documented case of as syngas from wood through a gasification process. wastewater treatment plant sludge AD. Three different Gasification is a thermochemical process where biomass is converted into a mixture of gases that compositions; (1) pure hydrogen, (2) 86 vol.% H2, 7 contains H2, CO and CO2 (Bridgwater, 2003). The vol.% CO and 7 vol.% CO2, and (3) 44.4 vol.% H2, 33.3 produced syngas can be directly fed into the AD reactor vol.% CO and 22.2 vol.% CO2 were used for a first set of simulations testing process limitations. The second for methane production, making AD a method to set of simulations were used to find out how much convert syngas into methane. methane production can be increased for the given case Adding syngas to AD can potentially have significant environmental impact on organic waste handling. It can if syngas composition is optimized. The CH4 production for instance be a way to obtain more bio-fuel as methane can be increased by 33 % by adding H2 (1) and was from AD than what is obtainable from the wet organic limited by pH going too high. Biogas CH4 content wastes currently used as feed for biogas production. reached 92 % at this limit. The H2-rich syngas addition This study can help estimate how much production can (2) reached 47 % CH4 production increase with 81 % increase and under which conditions. This approach CH4 content. The low H2 syngas case (3) produce more may also serve as a way to mineralize all organic matter biogas but the CH4 content is reduced to 42 %. There is a narrow syngas composition range for which methane in sludge by combining AD and thermal gasification. production can be increased by a factor >~ 2.7, limited Hydrogen can be used to upgrade the methane production directly in the reactor by increasing the by available nitrogen in the treated sludge. hydrogenotrophic methanogenesis (Luo & Angelidaki, Keywords: Anaerobic digestion, ADM1, Syngas 2013), which consumes hydrogen together with CO2 in addition, CH4 production, CO degradation the biogas, with methane as product (Luo & Angelidaki, 2012): H 4 H 22 CHCO +→+ 2 24 OH (1) 1 Introduction Many degradation paths for CO has been suggested, The concept of waste to energy from wet organic waste but experiments have shown acetogenesis to be like manure for biogas generation by Anaerobic dominating (Luo et al., 2012) under anaerobic condition Digestion (AD) is a prominent green technology since it at mesophilic temperatures. Acetogens utilize the CO reduces greenhouse gases and odors (Deublein & and yields acetate, CO2, cell material and unrecovered Steinhauser, 2011). Carbon (Mörsdorf et al., 1992): Anaerobic digestion is a biochemical process, where microbial activity comes into play and reduce complex 6.8 → CHCO 3COOH + 3.5 CO2 organic pollutant by extracellular (disintegration, (2) + + hydrolysis) and intracellular (acidogenesis, 0.4 biomass 0.9C unrecovered C acetogenesis, methanogenesis) (Fig. 1) to produce The reaction (Eq. 2) is added to the ADM1 model biogas (Batstone et al., 2002). The generated biogas which is the standard platform of modelling and consists of (55-75) % methane and (25-45) % carbon simulations of AD process developed by IWA in 2002. dioxide (De Mes et al., 2003). ADM1 model is a structured model that describes the biochemical (Fig.1) and physiochemical reactions that (1) Implementation of CO degradation in ADM1. are responsible for methane production (Batstone et al., (2) Simulating hydrogen alone or syngas as AD feed 2002). The biochemical reactions are the core of this supplements. model which includes disintegration of complex organic (3) Evaluation of syngas component effects on the material to carbohydrates, proteins, and lipids. These are AD reactor performance by adding different then hydrolysed into sugars, amino acids and long-chain ratios of H2/CO/CO2. fatty acids (LCFAs) which are further fermented into molecular hydrogen and volatile organic acids 2 Materials and Methods (acidogenesis). The acids are broken down to acetate and hydrogen (acetogenesis). The last step is the split of The ADM1 model was extended by adding CO acetate ions into methane and carbon dioxide degradation (Table 1 and 2). H2 or syngas was supplied (acetoclastic methanogenesis). The hydrogenotrophic as input to the reactor compartment. The simulations methanogenesis step (Eq. 1) also produces methane were based on a reported sludge digestion experiment when hydrogen reduces carbon dioxide (Batstone et al., with ADM1 simulations (Siegrist et al., 2002), to which 2002). The hydrogenotrophic methanogens are thus H2 or syngas was added in various amounts. Applicable already present in an AD reactor and can grow to handle supply range is assumed to be between zero and the level at which methane production fails. more hydrogen or syngas. If H2 is added in excess, it can remove so much CO2 (Eq. 3) that pH rise too high for 2.1 Syngas degradation in ADM1 efficient methanogenesis (Luo et al., 2012) ultimately causing failure of the reactor. The ratio of added H2/feed Syngas addition requires two new biochemical reactions load and effect of composition in the added syngas are to be added into the model. One is the uptake of carbon therefore evaluated here to evaluate syngas addition monoxide to acetate and the second is decay of carbon limitations. monoxide degrading organism. The parameters used for CO + H O ↔H CO ↔ H++ HCO- (3) uptake are in Table 1 and the rate equations and 2 2 2 3 3 stoichiometry coefficients are given in Table 2. The physiochemical processes are liquid-liquid mass transfer process (i.e. ion dissociation) and liquid-gas exchange (i.e. liquid-gas mass transfer) (Batstone et al., Table 1: Parameters used for uptake of CO. 2002). Inefficient syngas mass transfer can limit its Parameters Description units degradation in AD process due to the low solubility of km_CO_ac Maximum uptake rate for kg COD S -1 CO and H2 (Guiot, Cimpoia, & Carayon, 2011) which CO degrading organisms. kg COD X -1 can result in syngas loss to headspace. In this work it is d X_CO_ac CO degrading organisms. kg COD m-3 assumed that such loss is avoided by adding the gas Ks_CO_ac Half-saturation constant kg COD m-3 through a membrane by diffusion. for CO degradation (same as for H2 degradation). I_ph_CO_ac pH inhibition of CO to - acetate degrading organisms (same as for propionate degradation). I_H2_CO_ac Hydrogen inhibition for - CO to acetate degrading organism. kdec_x_CO_ac Decay rate for CO d-1 degrading organisms Y_CO_ac Yield of biomass on the kg COD kg-1 uptake of CO to acetate. COD S_CO Total carbon monoxide. kg COD m-3 KH_CO Non-dimensional Henry's M (liq) M-1 law constant for CO (gas) Figure 1. Systematic representation of anaerobic digestion process showing biochemical reactions described in Table 2: Uptake rate of CO and decay rate of CO ADM1 model (Batstone et al., 2002). degrading organism in the model. The purpose of this study is to evaluate effects of Dynamic Rate equation process syngas composition and quantity on methane production uptake_CO_ac km_CO_ac*X_CO_ac*S_CO/(Ks_CO_ and biogas composition, when added to an AD reactor ac+S_CO)*I_ph_CO_ac*I_H2_CO_ac* running on sludge. The ADM1 model implemented in I_NH_limit the AQUASIM software is applied and includes: decay_CO_ac X_CO_ac*kdec_x_CO_ac DOI: 10.3384/ecp17138114 Proceedings of the 58th SIMS 115 September 25th - 27th, Reykjavik, Iceland Estimations of km and Y for CO uptake are based on reported experimental results (Mörsdorf et al., 1992) 6 and the observed stoichiometric reaction of CO utilization (Eq. 4). 5 ) 4 1 CHCO 6.8 → CHCO 3COOH + 3.5 CO 2 - (4) d 3 + 0.4 biomass + 0.9C dunrecovere C 3 (m 2 The unrecovered carbon is here assumed to be Feed flow 1 divided between acetate and CO2 in the same way as the recovered part observed by Mörsdorf et al. (1992), 0 0 10 20 30 40 50 according to (Eq. 5). Time (d) CH 1.15CO 6.8 →1.15CO CH COOH + 4.1 CO 3 2 (5) Figure 2: Sludge feed flow to the pilot reactor (Wang et + 0.4 biomass C al., 2013). Eq.5 is further converted into COD basis (Eq. 6) The feed composition of amino acid, fatty acid, sugar using 5 mole carbon per mole biomass and 160 g COD and composite organic material are in Table 3. mole-1 for biomass (Batstone et al., 2002). Table 3: Feed composition (Wang et al., 2013). 73.6CO 108.8 → 73.6CO CH COOH + 0 CO 3 2 (6) Components in reactor feed Concentration + 12.8 biomass (kg COD m-3) Amino acids 4.2 From equation 6, it can be seen that biomass yield per Fatty acids 6.3 g COD of CO is obtained by: Monosaccharides 2.8 Composite material 10 12,8 Total 23.3 Y == 0,12 g COD biomass g 1- COD CO 108,8 The average feed flow the first 16 days is 1.61 m3 d-1 The relation between maximum uptake rate of -1 3 -1 max or 37.5 kg COD d .